HTML5 Web Audio API，从javax.sound移植并获得失真 [英] HTML5 Web Audio API, porting from javax.sound and getting distortion

问题描述

我需要动态生成音频（从wav或mp3文件生成不是一个选项）。幸运的是，新的WebAudio API（FF4和Chrome 13）提供了这项功能。

I have a requirement to generate audio on the fly (generating from wav or mp3 files is not an option). Luckily the new WebAudio API (FF4 and Chrome 13) provides this functionality.

我有一些Java代码，我正在移植到Javascript，如下所示：

I have some Java code i'm porting to Javascript that looks like this:

byte[] buffer = new byte[]{ 56, -27, 88, -29, 88, -29, 88, -29 ............ };
AudioFormat af = new AudioFormat(44100, 16, 1, true, false);
SourceDataLine sdl = AudioSystem.getSourceDataLine(af);
sdl.open(af, 1470 * 4); //create 4 frame audio buffer
sdl.start();
sdl.write(buffer, 0, buffer.length);

我正在尝试使用Web Audio API，但它极度失真。这是我在JS中使用的代码：

I'm trying to get this working with the Web Audio API, but it is extremely distorted. Here is the code i'm using in JS:

var buffer = [ 56, -27, 88, -29, 88, -29, 88, -29 ............ ];
var ctx = new webkitAudioContext();
var src = ctx.createBufferSource();
src.buffer = ctx.createBuffer(1, buffer.length, 44100);
src.buffer.getChannelData(0).set(buffer);
src.connect(ctx.destination);
src.looping = false;
src.noteOn(0);

这是我正在测试的.java文件：
http://gwt-nes-port.googlecode.com/svn/wiki/webAudioTests /Main.java

Here is a .java file that i'm testing with: http://gwt-nes-port.googlecode.com/svn/wiki/webAudioTests/Main.java

这是我正在测试的.js文件：
http://gwt-nes-port.googlecode.com/svn/wiki/webAudioTests/Main.js

And here is the .js file that i'm testing with: http://gwt-nes-port.googlecode.com/svn/wiki/webAudioTests/Main.js

有关javax.sound和Web Audio如何工作之间差异的任何提示，以及导致我的JS代码失真的原因是什么？

Any tips on the differences between how javax.sound and Web Audio work, and what is causing the distortion in my JS code?

推荐答案

感谢非常乐于助人的Google Chrome团队，我想出了这一点。以下是他们所说的：

Thanks to the very helpful Google Chrome team, I figured this one out. Here is what they said:

嗨Brad，看起来你的
样本数据超出了有效的
范围。对于此API，满量程
浮点PCM音频数据应
在-1.0范围内 - > +1.0

Hi Brad, it looks like your sample-data is outside of the valid range. For this API, the full-scale floating-point PCM audio data should be within the range -1.0 -> +1.0

也许你的数据值是16位
缩放（-32768 - > +32767）。

Perhaps your data values are 16bit scaled (-32768 -> +32767).

所以当我创建我的字节数组时，我需要确保所有内容都表示为小数。所以不是这样：

So when I create my byte array, I need to make sure everything is represented as a decimal. So instead of this:

byte[] buffer = new byte[]{ 56, -27, 88, -29, 88, ............ };

我真的需要这样的东西：

I really needed something that looked like this:

byte[] buffer = new byte[]{ 0.023, -0.1, 0.125, -0.045, ............ };

所以在我的代码中我添加了一些逻辑来将16位缩放值转换为适当的范围，如下所示：

So in my code I just added some logic to convert the 16bit scaled values to the appropriate range like this:

for(var i=0;i<buffer.length;i++) {
   var b = buffer[i];
   b = (b>0)?b/32767:b/-32768;
   buffer[i] = b;
}

现在音频表示为小数，不再听起来像是扭曲的重音金属歌曲。

Now the audio is represented as a decimal and no longer sounds like a distorted heavy metal song.

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